Re: shmat problem

[Doug McNaught <doug@mcnaught.org>]

Alex Riesen <fork0@users.sf.net> writes:

> Doug McNaught, Mon, Jan 06, 2003 17:36:39 +0100:
> > > You have to add SHM_REMAP to shmat flags (see definitions of SHM_ flags).
> > Hmm, the manpage (on RH7.3 at least) doesn't mention SHM_REMAP.  Nice
> > to know about it.
> 
> RH7.3 manpage is quiet old, btw.

Apparently so.

> Linux manpages-1.54 (Dec 30 2002):
> 
>    The  (Linux-specific) SHM_REMAP flag may be asserted in shmflg to indi-
>    cate that the mapping of the segment should replace any  existing  map-
>    ping  in  the  range starting at shmaddr and continuing for the size of
>    the segment.  (Normally an EINVAL  error  would  result  if  a  mapping
>    already  exists in this address range.)  In this case, shmaddr must not
>    be NULL.

Wouldn't the OP's code still (potentially) have problems? What if you
had:

char my_shared_area[2048];
int my_unshared_var;

...

   void *foo = shmat(id, &my_shared_area, SHM_REMAP);


Would my_unshared_var end up shared, since memory mappings have page
granularity?

-Doug